A thin film transistor (TFT) is broadly used as a switching element for display of a liquid crystal display, etc. A cross-sectional structure of a representative TFT is shown in FIG. 3. As shown in FIG. 3, TFT has a gate electrode, an insulator layer and an organic semiconductor layer in this order on a substrate and has a source electrode and a drain electrode formed at a prescribed interval on the organic semiconductor layer. The organic semiconductor layer forms a channel region and when a current flowing between the source electrode and the drain electrode is controlled by a voltage to be applied to the gate electrode, undergoes an ON/OFF operation.
Hitherto, this TFT has been prepared using amorphous or polycrystalline silicon. However, there was a problem that a CVD apparatus which is used for the preparation of TFT using such silicon is very expensive so that increasing in size of a display, etc. using TFT is accompanied by a significant increase of manufacturing costs. Also, since a process for fabricating amorphous or polycrystalline silicon is carried out at a very high temperature, the kind of a material which can be used as a substrate is limited, causing a problem that a lightweight resin substrate or the like cannot be used.
In order to solve such a problem, TFT using an organic material in place of amorphous or polycrystalline silicon is proposed. With respect to the fabrication method to be employed for forming TFT using an organic material, there are known a vacuum vapor deposition method, a coating method and so on. According to such a fabrication method, it is possible to realize increasing in size of a device while suppressing an increase of the manufacturing costs, and the process temperature which is necessary at the time of fabrication can be made relatively low. For that reason, in TFT using an organic material, there is an advantage that limitations at the time of selection of a material to be used for the substrate are few, and its realization is expected. TFT using an organic material has been eagerly reported, and, for example, Non-Patent Documents 1 to 20 can be enumerated.
Also, as the organic material to be used in an organic compound layer of TFT, so far as a p-type is concerned, multimers such as conjugated polymers, thiophenes, etc. (Patent Documents 1 to 5, etc.); metallophthalocyanine compounds (Patent Document 6, etc.); condensed aromatic hydrocarbons such as pentacene, etc. (Patent Document 7 and 8, etc.); and the like are used singly or in a state of a mixture with other compounds. Also, so far as a material of an n-type FET is concerned, for example, Patent Document 9 discloses 1,4,5,8-naphthalenetetracarboxyl dianhydride (NTCDA), 11,11,12,12-tetracyanonaphth-2,6-quinodimethane (TCNNQD), 1,4,5,8-naphthalenetetracarboxyl diimide (NTCDI), etc; and Patent Document 10 discloses phthalocyanine fluoride.
Furthermore, Patent Document 11 describes an invention regarding an organic semiconductor material including at least one structure in which aromatic hydrocarbon ring-containing or aromatic heterocyclic ring-containing groups are bonded to each other via an alkynylene group and having a molecular of not more than 2,000 and an organic electronic device using the same, enumerates compounds having a thiophene ring as a preferred structure and merely exhibits device properties regarding the compounds having a thiophene ring as the working examples. Also, Patent Document 12 describes an invention of thienothiophene derivatives, discloses compounds in which the thienothiophene derivative is bonded to benzene via acetylene and describes the utilization for a thin film transistor; however, it does not describe working examples thereof.
On the other hand, there is an organic electroluminescence (EL) device as a device similarly using electric conduction. However, the organic EL device generally forcedly feeds charges upon application of a strong electric field of 105 V/cm or more in the thickness direction of a ultra-thin film of not more than 100 nm; whereas in the case of the organic TFT, it is necessary to feed charges at a high speed over a distance of several μm or more in an electric field of not more than 105 V/cm, and accordingly, the organic material itself is required to become more conductive. However, the foregoing compounds in the conventional organic TFTs involved a problem in high-speed response as a transistor because the field effect mobility is low, and the response speed is slow. Also, the ON/OFF ratio was small. The terms “ON/OFF ratio” as referred to herein refer to a value obtained by dividing a current flowing between a source and a drain when a gate voltage is applied (ON) by a current flowing between the source and the drain when no gate voltage is applied (OFF). The terms “ON current” as referred to herein usually refer to a current value (saturated current) at the time when the current flowing between the source and the drain is saturated when the gate voltage is increased.    [Patent Document 1] JP-A-8-228034    [Patent Document 2] JP-A-8-228035    [Patent Document 3] JP-A-9-232589    [Patent Document 4] JP-A-10-125924    [Patent Document 5] JP-A-10-190001    [Patent Document 6] JP-A-2000-174277    [Patent Document 7] JP-A-5-55568    [Patent Document 8] JP-A-2001-94107    [Patent Document 9] JP-A-10-135481    [Patent Document 10] JP-A-11-251601    [Patent Document 11] JP-A-2004-179249    [Patent Document 12] JP-A-2003-73382    [Non-Patent Document 1] F. Ebisawa, et al., Journal of Applied Physics, Vol. 54, page 3255, 1983    [Non-Patent Document 2] A. Assadi, et al., Applied Physics Letter, Vol. 53, page 195, 1988    [Non-Patent Document 3] G. Guillaud, et al., Chemical Physics Letter, Vol. 167, page 503, 1990    [Non-Patent Document 4] X. Peng, et al., Applied Physics Letter, Vol. 57, page 2013, 1990    [Non-Patent Document 5] G. Horowitz, et al., Synthetic Metals, Vol. 41-43, page 1127, 1991    [Non-Patent Document 6] S. Miyauchi, et al., Synthetic Metals, Vol. 41-43, 1991    [Non-Patent Document 7] H. Fuchigami, et al., Applied Physics Letter, Vol. 63, page 1372, 1993    [Non-Patent Document 8] H. Koezuka, et al., Applied Physics Letter, Vol. 62, page 1794, 1993    [Non-Patent Document 9] F. Garnier, et al., Science, Vol. 265, page 1684, 1994    [Non-Patent Document 10] A. R. Brown, et al., Synthetic Metals, Vol. 68, page 65, 1994    [Non-Patent Document 11] A. Dodabalapur, et al., Science, Vol. 268, page 270, 1995    [Non-Patent Document 12] T. Sumimoto, et al., Synthetic Metals, Vol. 86, page 2259, 1997    [Non-Patent Document 13] K. Kudo, et al., Thin Solid Films, Vol. 331, page 51, 1998    [Non-Patent Document 14] K. Kudo, et al., Synthetic Metals, Vol. 102, page 900, 1999    [Non-Patent Document 15] K. Kudo, et al., Synthetic Metals, Vol. 111-112, page 11, 2000    [Non-Patent Document 16] Advanced Materials, Vol. 13, No. 16, 2001, page 1273    [Non-Patent Document 17] Advanced Materials, Vol. 15, No. 6, 2003, page 478    [Non-Patent Document 18] W. Geens, et al., Synthetic Metals, Vol. 122, page 191, 2001    [Non-Patent Document 19] Lay-Lay Chua, et al., Nature, Vol. 434, March 10 issue, 2005, page 194    [Non-Patent Document 20] C. D. Dimitrakopoulos, et al., IBM J. RES & DEV., Vol. 45, No. 1, page 11, 2001